Recently, not only optical equipment and digital cameras but cellular phones and other devices are equipped with cameras, and there is an increasing demand for low-cost lenses which are smaller in size and higher in performance. As a method for manufacturing such lenses, a hot mold press-forming method for preform material is widely employed in place of a conventional glass sheet polishing method. In this manufacturing method, a lens is formed by hot press-forming after arranging glass beads, preform material, between metal molds having same curved surfaces as those of the lens. In this case, for increasing the shape accuracy of the lens to be finally formed, it is desirable that the glass beads, preform material, be as spherical as possible and free from defects such as internal bubbling. The reasons for this are as follows. Namely, when glass beads are excellent in sphericity, no bias load will be applied to the glass beads in the initial state of pressing in metal molds, and as a result, it is possible to enhance the accuracy of the lens formed by press-forming.
As a method for producing such glass beads as preform material, available is a method of grinding and polishing glass into spheres. However, in the glass grinding and polishing method, there is a great loss of glass material, and also, much time is required for producing glass beads and it is unable to improve the productivity.
Accordingly, in the Japanese Laid-open Patent H2-14839, disclosed is a method for producing glass beads in which glass is melted in a platinum pot, and the molten glass is dripped into air from a platinum nozzle connected to the pot, and the dripped molten glass is cooled in a state of being floated by a flow of air jetted above the receiving mold provided with an air jetting orifice at the bottom thereof. Also, in the Japanese Laid-open Patent H10-291824, disclosed is a method of cooling and hardening drops of molten glass thrown into a solution having cleaning effects. In this method, alcohol or a mixture of alcohol and water is used as the solution, and it is disclosed that glass beads with excellent sphericity can be manufactured while shortening the dripping distance.
However, in the case of producing a glass bead by cooling while making it floated in air, it is relatively difficult to cool a drop of molten glass in a state of being precisely floated by air flow. As a result, the drop of molten glass may sometimes come into direct contact with the inner surface of the receiving mold. In that case, there arises a problem such that the drop of molten glass hardens as it is deformed or strained. Further, when a plurality of drops of molten glass are dripped into same receiving mold, the drops of molten glass come into contact with each other to be broken, cracked or hardened, and it is necessary to take out the glass beads before dripping the next molten glass. Accordingly, there is a problem of poor productivity.
Also, in the method of producing glass beads by cooling drops of molten glass thrown into alcohol or the mixture of alcohol and water, defects such as breaking of glass beads and vacuum bubbling are liable to take place, and the yield as preform material cannot be improved, and it is relatively difficult to manufacture highly accurate lenses at low costs.